Why Does Lettuce Bolt and What Can I Do About It?

Lettuce growing garden
Jean Jones/ Flickr/ CC BY 2.0

Vegetable gardeners often talk about their plants bolting. This simply means that the plant sends up a flower stalk and goes to seed. Generally when plants flower it is considered a good thing, but vegetables grown for their leaves, like lettuce, spinach, cabbage and other cole crops, the flavor turns bitter and the leaves tend to get smaller and tougher, making them inedible.

Bolting is very common in cool season greens, like arugula and lettuce and spinach, mentioned above, but many other plants, for example beets and broccoli and herbs like cilantro, basil and dill, will bolt too.

 

Why Does Lettuce Bolt and What Can I Do About It?

Bolting tends to happen when the temperature heats up, but researchers say that it is actually the longer hours of daylight that cause bolting. In one study, they covered some of the control plants during the day for different amounts of time and only the plants left exposed to a full day of sunlight bolted.

They also tried growing lettuce at high temperatures (90 F.), but with only 8 hours of sunlight and no plants bolted. In greenhouse studies, lettuce grown in the short days of January didn't bolt until about 135 days after planting, while those sown in July bolted in only 90 days.

Heat may be a factor in bolting, if it occurs when the plants are nearing maturity. Dry conditions may also contribute to it. Plants that feel threatened by harsh temperatures will often go to seed. Even exposure to cold while the plants are seedlings can play a role.

If lettuce seedlings are exposed to 40 – 50 F. temperatures for several days in a row, they will start forming flower buds then, although the flower stalk won't shoot up until the weather warms. Still, day length has been found to be the major culprit responsible for bolting.


What Can You Do to Prevent Bolting?

Not much.

Plants that bolt tend to prefer the cooler seasons and making them happy in the heat of summer will take work.

Some cultivars will be quicker to bolt than others. You will often see seeds touted as "slow to bolt", but since a variety of conditions can cause bolting, there is no guarantee. However if you do have trouble growing lettuce, for example, in the summer, give some of the cultivars a try. Many will have names that hint at their bolt resistance, like 'Slobolt' and 'Summer Bibb', but many of the older varieties seem just as hardy. I've read that crisp head lettuces in the romaine or cos group are the slowest to bolt and loose leaf the quickest, but I haven't had that experience in my garden.

Many gardeners, including me, have had success growing lettuce throughout the summer by planting it in a shaded spot in the garden. You can tuck it behind or under taller plants or grow it in pots that can be moved to a shadier site. Regular watering helps to keep the soil cool and the leaves succulent.

A trick I like for starting seeds in the summer is to thoroughly soak the area to be planted about 2-3 days before sowing. Cover the damp soil with a wide board and lift it and repeat this process daily, if the weather is particularly hot and dry.

Within a couple of days, the soil under the board will be cooler than the surrounding soil. Sow your seeds, water, and cover again with the board. Check daily for signs of germination. At the first sight of green sprouts, remove the board.

Lettuce is the biggest challenge, when it comes to bolting, but if you are having trouble with other vegetables, the same tips I've given for keeping your lettuce growing can be applied. More Growing Tips for Vegetables and Herbs that Bolt:

 

Source: Journal of the American Society for Horticultural Science